说明：  利用PA口的第二功能将PA0口（即ADC的0通道）输入的模拟量转变为数字量，通道的参考源选 择AVCC，并且数据采用右对齐方式，转换方式为每1秒启动一次转换，转换结果由实验台上 的七段数码管显示
(PA I use the second feature will PA0 mouth (that is, the 0-channel ADC) analog input into a digital volume, channel selection reference source AVCC, and adopt the right alignment of data, conversion means for every 1 seconds to start a conversion , conversion results from the experimental stage of the Seven-Segment LED display)